1. Field of the Invention
The present invention relates to a harmonic suppression resonator for suppressing, in a circuit using high frequency signals, a harmonic component having a different frequency from a fundamental wave frequency. The present invention also relates to a high-frequency device such as a harmonic propagation blocking filter, radar apparatus or the like, which comprises the harmonic suppression resonator.
2. Description of the Background Art
Conventionally, for the purpose of efficient use of radio wave resources, high-frequency devices are regulated and recommended so as not to cause unnecessary radiation in a frequency band which is remote from a frequency band used by the high-frequency devices.
Japanese Laid-Open Patent Publication No. 2004-274341 (hereinafter, referred to as Patent Document 1) describes a band-pass filter for improving a spurious characteristic of a microwave generating source.
FIG. 1 shows a structure of a waveguide band-pass filter of Patent Document 1. In the waveguide band-pass filter, waveguide resonators 1001a and 1001b, which have TE102 mode that is a fundamental mode of a rectangular waveguide, are provided so as to be connected to each other in a direction in which an electric field component E is orthogonal to a main propagation direction of TE10 mode that is a fundamental mode of a rectangular waveguide. The waveguide resonators 1001a and 1001b are connected such that a wide face, which is one of waveguide walls, of each waveguide resonator, is connected to the wide face of the other waveguide resonator so as to form a two-part structure. A turnaround section 1005 is provided at the connection between both the waveguide resonators 1001a and 1001b, which connection is a part of the connected waveguide resonators 1001a and 1001b. A coupling hole 1002a for coupling the waveguide resonators 1001a and 1001b is provided at a waveguide wall 1006 of the turnaround section 1005, which waveguide wall 1006 is formed by the wide waveguide faces dividing the waveguide resonators 1001a and 1001b. Input/output coupling holes 1003a and 1003b, which are respectively formed at one end and the other end of the connected waveguide resonators 1001a and 1001b, are separated from each other by the waveguide wall 1006 formed with the wide faces of the waveguide resonators 1001a and 1001b, and do not couple with each other. Input/output waveguides 1004a and 1004b are respectively connected, in a direction orthogonal to an electric field component, to the input/output coupling holes 1003a and 1003b that are respectively formed at one end and the other end of the connected waveguide resonators 1001a and 1001b. 
Another technique for suppressing unnecessary radiation contained in radio waves radiated from a radar using a large amount of power, is disclosed by Japanese Laid-Open Patent Publication No. 2007-81856 (hereinafter, referred to as Patent Document 2).
For a transmitter tube of a shipboard radar, a magnetron is used. The magnetron basically oscillates in π mode to generate a microwave having a fundamental wave frequency. At the same time, however, a frequency component of π-1 mode and a frequency component of a frequency-doubled wave (i.e., a second harmonic) occur as unnecessary radiation. In Patent Document 2, in order to suppress this unnecessary radiation, a rotary joint of a pedestal section is used, in which a spurious suppression filter (LPF) is provided in a coaxial tube on a central axis of the rotary joint.
In general, in a high-frequency device using a waveguide as a transmission path, a waveguide resonator is provided as a filter in order to allow only a fundamental wave component to propagate.
Further, Japanese Laid-Open Patent Publications No. 2004-274341 (hereinafter, referred to as Patent Document 3) and No. 2007-81856 (hereinafter, referred to as Patent Document 4) disclose techniques in which metallic blocks, which are obtained from dividing a metallic block along a longitudinal plane thereof, are combined to form a waveguide. Although this type of waveguide has advantages in manufacturing, it is necessary to provide a countermeasure for radio wave leakage (electrical loss) from a gap between planes, which face each other, of the combined metallic blocks. Patent Document 3 proposes to interpose a soft metallic foil between a metallic block, on which a waveguide groove is formed, and a metallic block, which covers the groove. In this manner, a gap between portions, which face each other, of the metallic blocks is eliminated by a tight contact between the metallic foil and the metallic blocks. Patent Document 4 proposes to silver-plate a vicinity of a groove of a plane of one metallic block, which plane faces a plane of the other metallic block, or to form a protrusion by using a metallic block or a different member, whereby a gap between the facing planes of the blocks is eliminated.
As mentioned above, a magnetron is used for a transmitter tube of a shipboard radar. The magnetron basically oscillates in π mode to generate a microwave having a fundamental wave frequency. At the same time, however, a frequency component of π−1 mode and a frequency component of a frequency-doubled wave (i.e., a second harmonic) occur as unnecessary radiation.
In a structure comprising a waveguide resonator as a filter, if a waveguide filter, which resonates in, for example, TE101 mode of a rectangular waveguide, is provided, not only a fundamental wave is transmitted but also a second harmonic is transmitted since the waveguide filer also resonates in TE202 mode. For this reason, it has been impossible to use a waveguide filter as a harmonic propagation blocking filter. This is described below using FIG. 2.
FIG. 2 shows a structure of a conventional waveguide resonator that does not have a harmonic-suppressing function. FIG. 2(A) is an external perspective view of the conventional waveguide resonator. Basically, the waveguide resonator has a shape which is formed in the following manner: a rectangular waveguide is cut such that wide planes thereof become square planes; and front and rear openings thereof are closed using conductive materials.
FIG. 2(B) schematically shows electromagnetic field distribution of a fundamental wave. FIG. 2(D) schematically shows electromagnetic field distribution of a second harmonic. Here, solid arrows represent lines of electric force at a given moment, and dot marks and cross marks represent directions of magnetic fields. In this manner, electromagnetic field intensity distribution is represented.
FIG. 2(C) shows, in relation to the electromagnetic field distribution of the fundamental wave, intensity distribution of electric field energy and magnetic field energy. FIG. 2(E) shows, in relation to the electromagnetic field distribution of the second harmonic, intensity distribution of electric field energy and magnetic field energy. In these diagrams, EF represents a region where the electric field energy is dominant, and MF represents a region where the magnetic field energy is dominant.
As shown herein, the waveguide resonator that resonates in the TE101 mode also resonates in the TE202 mode. Therefore, the second harmonic in the case where the fundamental wave is in the TE101 mode, cannot be suppressed.
Accordingly, even if the waveguide band-pass filter disclosed by Patent Document 1 is used in order to suppress the aforementioned unnecessary radiation, there is a problem that an effect to suppress the second harmonic component, which is crucial, is low.
In such a structure as disclosed in Patent Document 2 where a low-pass filter is used, harmonic components can be suppressed over a relatively wide frequency band within a frequency band that is higher than a fundamental wave frequency band. However, there is a problem that an attenuation characteristic in the frequency band higher than the fundamental wave frequency band is not steep, and an effect to suppress the second harmonic, which is crucial, is low.
Further, in the structure comprising a waveguide resonator as a filter, if a waveguide filter, which resonates in, for example, TE□101 mode of a rectangular waveguide (hereinafter, simply referred to as “TE101 mode”), is provided, not only a fundamental wave is transmitted but also a second harmonic is transmitted since the waveguide filter also resonates in TE□202 mode (hereinafter, simply referred to as “TE202 mode”). For this reason, it has been impossible to use a waveguide filter as a harmonic propagation blocking filter. This is described below with reference to FIG. 2.
FIG. 2 shows a configuration of a conventional waveguide resonator that does not have a harmonic-suppressing function. FIG. 2(A) is an external perspective view of the conventional waveguide resonator. Basically, the waveguide resonator has a shape which is formed in the following manner: a rectangular waveguide is cut such that wide planes thereof become square planes; and front and rear openings thereof are closed using conductive materials.
FIG. 2(B) schematically shows electromagnetic field distribution of a fundamental wave. FIG. 2(D) schematically shows electromagnetic field distribution of a frequency-doubled wave of the fundamental wave. Here, solid arrows represent lines of electric force at a given moment, and dot marks and cross marks represent directions of magnetic fields. In this manner, electromagnetic field intensity distribution is represented.
As shown herein, the waveguide resonator that resonates in the TE101 mode also resonates in the TE202 mode. Therefore, the second harmonic in the case where the fundamental wave is in the TE101 mode, cannot be suppressed.
Further, in Patent Document 3, since the soft metallic foil is used, the foil needs to be handled carefully, and it is questionable whether flatness or the like of the foil can be maintained in the long term. Thus, the technique disclosed in Patent Document 3 is not sufficient in terms of workability and reliability. Still further, in Patent Document 4, a new problem arises in relation to flatness of a surface of the formed protrusion, and thus, there is a limit to completely eliminate the gap.